In copper alloys, a Cu—Zr based copper alloy has been known as an alloy system having a high conductivity of 75% IACS or more. A Cu—Zr based copper alloy can achieve a strength level with high practical utility (for example, a tensile strength of approximately 450 MPa or more) as a current-carrying component, such as a connector, while retaining the aforementioned high conductivity, by controlling the final degree of working and the like. Furthermore, practical stress relaxation resistance characteristics (for example, a stress relaxation ratio of 25% or less at 200° C. for 1,000 hours) that are practical in various purposes can also be imparted thereto. However, in order to impart simultaneously a high conductivity and high stress relaxation resistance characteristics to the alloy system while enhancing the strength thereof, there have been many restrictions, for example, the contents of the third elements other than Zr are necessarily strictly limited. Therefore, for achieving a copper alloy that has a conductivity, a strength, and stress relaxation resistance characteristics at high levels, for example, a conductivity of 75.0% IACS or more, a tensile strength of 450 MPa or more, and a stress relaxation ratio of 25% or less at 200° C. for 1,000 hours, there have been factors increasing the cost, for example, inexpensive general scraps containing Sn are difficult to use. Moreover, there have been considerable restrictions in the production process.
PTL 1 describes a technique of improving a creep resistance of a copper alloy by combined adding Zr and others. However, the example of an alloy containing Sn added thereto (Example No. 9) has a low conductivity of 43% IACS, and the high conductivity inherent to the Cu—Zr based copper alloy is impaired.
PTL 2 describes a copper alloy improved in Young's modulus and stress relaxation resistance characteristics. The example of an alloy containing Zr and Sn (Example 2-9 of invention shown in Table 2) has a low conductivity of 48.1% IACS and a not so high strength level.
PTL 3 describes a technique of improving a strength and bending workability by subjecting a Cu—Zr based alloy having a high conductivity to a rolling. The example of an alloy containing Zr and Sn (Example No. 2) achieves a conductivity of 86% IACS and a tensile strength of 530 N/mm2. However, there is no teaching about the stress relaxation resistance characteristics. According to the investigations made by the present inventors, sufficient improvement of the stress relaxation resistance characteristics cannot be expected by the measures described in PTL 3 (see Comparative Example 13 shown later).
PTL 4 describes a technique for providing a copper alloy that is difficult to cause deformation of a lead of a lead frame and has a short period of time required for stress relief annealing after a press working. While various elements that are capable of being added are exemplified, there is no specific example of combined addition of Zr and Sn. Furthermore, it is difficult to provide stably a high conductivity of 75.0% IACS by the technique.
PTL 5 describes a technique of providing a high conductivity and a high strength by adding Cr and the third elements, such as Zr and Sn. However, the stress relaxation ratio is from 14 to 19% under condition of 150° C.×1,000 hours, and further improvements thereof are demanded depending on purposes.
PTL 6 describes a technique of improving a bending deflection coefficient of a Cu—Zr—Ti based copper alloy. An example of combined addition of Sn is disclosed (Example 21 of invention in Table 1), but the tensile strength thereof is as low as 386 MPa.
PTL 7 describes a technique of improving bendability and drawability of a Cu—Zr—Ti based copper alloy. An example of combined addition of Sn is disclosed (Example 16 of invention in Table 1), but there is no teaching about improvement of stress relaxation resistance characteristics.
PTL 8 describes a technique of providing high bending workability and a high spring elastic limit for a Cu—Zr based copper alloy by making a structure state with a KAM value of from 1.5 to 1.8° within the crystal grains. However, there is not description about the addition of Sn, and there is not teaching about a measure for enhancing the stress relaxation resistance characteristics.